Reducing speckle in an excimer light source

Abstract

A method includes: producing a light beam made up of pulses having a wavelength in the deep ultraviolet range, each pulse having a first temporal coherence defined by a first temporal coherence length and each pulse being defined by a pulse duration; for one or more pulses, modulating the optical phase over the pulse duration of the pulse to produce a modified pulse having a second temporal coherence defined by a second temporal coherence length that is less than the first temporal coherence length of the pulse; forming a light beam of pulses at least from the modified pulses; and directing the formed light beam of pulses toward a substrate within a lithography exposure apparatus.

What is claimed is: 
     
       1. An apparatus comprising:
 an optical source configured to produce a light beam made up of pulses and having a wavelength in the deep ultraviolet range, each pulse having a first temporal coherence defined by a first temporal coherence length and each pulse being defined by a pulse duration; 
 a coherence reduction system in the path of the light beam of pulses and configured to, for each of the pulses in the light beam, modulate an optical phase over the pulse duration of the pulse to produce a modified pulse having a second temporal coherence defined by a second temporal coherence length that is less than the first temporal coherence length of the pulse; and 
 a beam directing apparatus in the path of a light beam of pulses formed from the modified pulses, the beam directing apparatus configured to direct the light beam of pulses formed from the modified pulses toward a substrate within a lithography exposure apparatus. 
 
     
     
       2. The apparatus of  claim 1 , wherein the second temporal coherence length is between 50-95% of the first temporal coherence length. 
     
     
       3. The apparatus of  claim 1 , wherein the optical source comprises:
 a first stage light source configured to produce a seed light beam made up of seed light beam pulses; and 
 a second stage optical amplifier configured to receive the seed light beam pulses and produce a light beam made up of amplified pulses. 
 
     
     
       4. The apparatus of  claim 3 , wherein the first stage light source includes a solid state gain medium. 
     
     
       5. The apparatus of  claim 1 , wherein the optical source comprises a solid state gain medium. 
     
     
       6. The apparatus of  claim 1 , wherein the coherence reduction system includes a two-dimensional array of phase modulators positioned within a beam homogenizer. 
     
     
       7. The apparatus of  claim 6 , wherein the two-dimensional array of phase modulators is also configured to, for each pulse, reduce a spatial coherence of the pulse so that the modified pulse has a second spatial coherence that is less than the spatial coherence of the pulse. 
     
     
       8. The apparatus of  claim 1 , further comprising an optical temporal pulse stretcher configured to increase a duration of the modified pulses. 
     
     
       9. The apparatus of  claim 1 , wherein the coherence reduction system comprises a Pockels cell including a medium through which the light beam of pulses passes, and modulating the optical phase over the pulse duration of the pulse comprises modulating an index of refraction of the medium of the Pockels cell. 
     
     
       10. The apparatus of  claim 1 , wherein the coherence reduction system comprises a single phase modulator. 
     
     
       11. The apparatus of  claim 1 , wherein the coherence reduction system is configured to produce, for each pulse of the light beam, the modified light beam pulse having a bandwidth that is greater than a bandwidth of the light beam pulse. 
     
     
       12. The apparatus of  claim 1 , wherein the coherence reduction system comprises a two-dimensional array of phase modulators optically arranged in parallel with each other. 
     
     
       13. The apparatus of  claim 12 , wherein the coherence reduction system is between two lenslet arrays of a beam homogenizer, wherein each phase modulator aligns with a pair of lenslets. 
     
     
       14. The apparatus of  claim 12 , wherein the coherence reduction system is near a pair of lenslet arrays of a beam homogenizer, wherein each phase modulator aligns with a pair of lenslets. 
     
     
       15. The apparatus of  claim 1 , further comprising a control system in communication with the coherence reduction system, the control system configured to:
 determine whether a bandwidth of the modified pulse is within a range of a target bandwidth; and 
 if it is determined that the bandwidth of the modified pulse is outside the range of the target bandwidth, then adjust a frequency at which the optical phase over the pulse duration of the pulse that produces the modified pulse is modulated. 
 
     
     
       16. A photolithography exposure apparatus comprising:
 an optical source configured to produce a light beam made up of pulses and having a wavelength in the deep ultraviolet range, each pulse having a first temporal coherence defined by a first temporal coherence length and each pulse being defined by a pulse duration; 
 an optical arrangement comprising an illumination module, a reticle, and a projection stage aligned along an optical axis with a wafer stage; and 
 a coherence reduction system in the path of the light beam of pulses and configured to, for each of the pulses in the light beam, modulate an optical phase over the pulse duration of the pulse to produce a modified pulse having a second temporal coherence defined by a second temporal coherence length that is less than the first temporal coherence length of the pulse; 
 wherein a wafer positioned at the wafer stage is configured to receive a light beam of pulses formed from the modified pulses. 
 
     
     
       17. The photolithography exposure apparatus of  claim 16 , wherein the coherence reduction system is within the optical arrangement. 
     
     
       18. The photolithography exposure apparatus of  claim 16 , wherein the coherence reduction system is within the optical source. 
     
     
       19. The photolithography exposure apparatus of  claim 16 , further comprising an optical temporal pulse stretcher configured to increase a duration of pulses that pass through it, the optical temporal pulse stretcher in the path of the light beam of pulses between the optical source and the optical arrangement. 
     
     
       20. The photolithography exposure apparatus of  claim 19 , wherein the coherence reduction system is within the optical temporal pulse stretcher.